cd ~/.ssh/

ssh-keygen -t rsa -b 4096

Output:

Generating public/private rsa key pair.
Enter file in which to save the key (/c/Users/odefta/.ssh/id_rsa): id_rsa_odcent8
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in id_rsa_odcent8
Your public key has been saved in id_rsa_odcent8.pub

ssh-copy-id -i ~/.ssh/id_rsa_odcent8.pub root@remote_ssh_server

Connect to remote_ssh_server and edit /etc/ssh/sshd_config:

vi /etc/ssh/sshd_config
PubkeyAuthentication yes

systemctl restart sshd

On the host machine (exit from the remote one):

Edit ~/.ssh/config file and add:

vi ~/.ssh/config

Host remote_ssh_server
    User root
    IdentityFile ~/.ssh/id_rsa_odcent8

Now you can connect without password:

ssh root@remote_ssh_server